[whatwg] Enabling LCD Text and antialiasing in canvas

[Robert O'Callahan]

On Sat, Feb 16, 2013 at 4:35 AM, Stephen White <senorblanco at chromium.org>wrote:

> Even with these constraints, I don't think we can guarantee that it's safe
> to use LCD AA text. Once you've drawn with LCD AA text, even if it's safe
> at the time of drawing, there's no guarantee that it will be safe later.
>  For instance, if you then drawImage() that canvas rotated into another
> canvas, or even just full-page-zoom it, you'll see colour fringing. Or
> apply CSS 2D or 3D transforms. There are also existing apps which use
> canvas for 2D text glyphs, and then transform and place them rotated in
> WebGL. Those will show colour fringing.


This came up on the list earlier.

Even within canvas, there may be a way to break it if the LCD AA text is
> drawn first and a dest-alpha compositing mode is used overtop (I haven't
> verified this though).


I don't think that's a problem. All destination alpha values will be 1 even
after the subpixel AA test is painted, and you'll treat it as normal.

So I'm starting to think that LCD AA text really has to be opt-in, to avoid
> breaking existing content. By opting it, you're agreeing that these
> artifacts are acceptable for your app. For example, you know that even if
> you're going to do a canvas-to-canvas draw, you're always going to draw at
> 1:1 scale and no rotation.
>

It's difficult to know that on a mobile browser or any other browser where
you have some kind of fast zoom UI.

The suggestion on the list earlier was to keep two versions of the canvas
buffer: one with grayscale AA, another with subpixel AA, and composite with
the subpixel AA buffer when we can do that safely, otherwise use the
grayscale AA version. In many implementations there would be a performance
hit for this, so it would make sense to have authors opt-in to that
performance hit.

Rob
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